Recent reports have suggested substantial correlations between certain EEG evoked potential (EP) parameters and intelligence test scores in young subjects falling in the age range of 15-22 years. The present study involved the investigation of relationships between intelligence measures and auditory EP parameters. The study was designed to ascertain whether the findings could be generalized to older subjects. EP measures and intelligence test scores were obtained from 99 adult subjects participating in the Baltimore Longitudinal Study of Aging. There was a tendency for older subjects to fall asleep during the EEG recording session. Consequently, the data were analyzed separately for subjects who stayed awake and for those who slept. Separate analyses were also performed for male and female subjects. The main finding is that correlations of the order of magnitude reported in the earlier studies referred to above were only found in the data obtained from subjects who slept during the EEG recording session. These subjects tended to be older than the subjects who did not sleep. Correlations obtained in the data of the subjects who did not sleep were small in contrast. The results have been interpreted in terms of the Yerkes-Dodson law which postulates that "arousal" is linked to performance and that the relationship takes the form of an "inverted-U" curve. It is suggested that both the very young subjects tested in the earlier studies and the older subjects who slept in the present study are functioning at a lower level of arousal. At this lower level of arousal the relationship between arousal-related EEG parameters and performance is nearly linear since it corresponds to the ascending limb of the inverted-U curve. Subjects falling in the intermediate range of ages are more highly aroused, it is suggested, and as a consequence their data relates to the flat middle section of the arousal/performance curve, if not to the descending high arousal limb.